1. Field of the Invention
The invention relates to the field of sealing systems for inlet or outlet ports of valves, such as Schrader-type valves and the like and, in particular, to an all metal sealing assembly that provides for a sealing gasket to seated about the periphery of the port by the application of only a compression load.
2. Description of Related Art
Satellites in orbit and deep space probes must often operate for years on limited supplies of fuel and other pressurized liquids and gases. Thus providing zero or extremely low leakage rates on system components thereof such as valves, lines and tanks is a necessity. This is particularly important for the valves used to fill tanks. Port sealing assemblies using elastomeric seals such as O-rings often prove unacceptable because they deteriorate over a long period of time especially in a space environment. In addition, when the tank contains a gas such as Hydrogen or Helium, they will slowly migrate through elastomeric seals. Such a leakage rate, which would be ignored on a ground based tank or on an aircraft, is unacceptable on a satellite or space probe where a fixed supply of gas must last up to fourteen years.
Metal to metal seals are preferred for use with such applications, for example, metal O-rings and the like. However, these require mating surfaces with very fine finishes. Another standard fitting design incorporates a male conical sealing surface on the port, which includes external treads. A hollow cap having a closed first end with an aperture therethrough and an open second end. The cap further includes internal threads adapted to mate with the external threads of the port and external wrenching flats. A plunger having a circular head portion with a female conical sealing surface is adapted to fit within the hollow cap and includes a shaft portion extending out of the aperture. A conical shaped gasket made of soft metal, usually copper or sometimes silver plated nickel, is positioned between the two sealing surfaces. When the cap is tightened, the gasket forms a seal. However, while this fitting is reusable, it depends upon the friction between the gasket and the conical surfaces to prevent rotation of the plunger or gasket. In addition, it depends upon highly polished sealing surfaces.
To eliminate the need for such super fine fines, knife-edge seals are used that create high-localized loads on the gasket. For example, U.S. Pat. No. 4,988,130 "Metal Seal Flange Assembly" by K. Obara, et al. discloses a bolted flange joint wherein a metal ring shaped gasket made of copper is sandwiched between two flanges having circular knife-edges. When the flanges are bolted together, the knife-edges locally deform the gasket creating a highly stressed joint at the knife-edge to gasket interface. This improves the seal's effectiveness. However, the dissimilar metals (flanges versus gaskets) can lead to leakage when used in extreme environments. For example, at cryogenic temperatures, the greater shrinkage of a copper gasket used with steel flanges can lead to relaxation of the joint.
On circular shaped ports that have screw-on caps, a soft metal gasket, typically made of lead or copper, is installed in the cap. When the cap is torqued down, the peripheral edge of the port acting locally deforms the gasket forming a seal. However, the combination of surface shear and compression loading on the gasket can cause micro-sized leakage paths. This is especially true if the gasket has even minor imperfections to start with. Furthermore, in standard applications of this sort wherein the gasket is often a washer (has a hole in the middle) there exists a second potential leak path.
Thus, it is a primary object of the invention to provide a sealing assembly for a port that has extremely low leakage rates.
It is another primary object of the invention to provide a sealing assembly for a port that uses metal sealing gaskets.
It is a further object of the invention to provide a sealing assembly for a port that uses metal sealing gaskets that engage a knife-edge located on the periphery of the port.
It is a still further object of the invention to provide a sealing assembly for a port that uses metal sealing gaskets that engage a knife edge located on the periphery of the port that prevents rotation of the gasket as it seals against the knife edge on the port.
It is an additional object of the invention to provide a sealing assembly that employs similar metals to mitigate the effects of extreme temperatures.